The earpieces of earphones or other audio or multimedia devices configured to be worn by a user, such as separate (e.g., left and right side) wireless or wired earbuds, or earpieces of headphones, may include components such as an acoustic driver that generates a sound wave to be heard by the user, and one or more microphones. For example, for earphones that use active noise reduction, the microphones within an earpiece may include one or more feedback microphones that detect sound within a controlled acoustic environment as part of a feedback loop, and one or more feedforward microphones that detect noise external the controlled acoustic environment to further aid in noise reduction. The (unknown) sensitivity of the microphones may be calibrated using testing procedures performed during manufacture of the earphones. For example, earphones may be placed on an “artificial head” that includes microphones having a known sensitivity inside “artificial ears” to measure the level of a test audio signal that arrives at the artificial ear when an active noise reduction function of the earpieces is being tested.